Automobiles and rail cars, which use electricity as an energy source, require a high voltage in order to drive their electric motors. Therefore, a plurality of storage battery cells are connected in series to configure a storage battery module, and further, a plurality of such storage battery modules are connected in series to configure a battery pack, and the battery pack is used.
Such storage battery cells have so-called battery characteristics, such as output voltage, and charge and discharge characteristics, the battery characteristics being different from each other, due to manufacturing variations, etc. Therefore, the storage battery modules, each being an assembly of those storage battery cells, also have battery characteristics different from each other.
When charging a battery pack including storage battery modules each having different battery characteristics, a storage battery module having a high output voltage and being charged more than the others reaches a charge limit earlier. If further continuing to charge this storage battery module, the storage battery module is overcharged, resulting in deterioration of the battery characteristics. Therefore, it is necessary to stop charging when the storage battery module having the highest output voltage and being the most charged in the battery pack reaches the charge limit. In this case, it is not possible to continue to charge the other storage battery modules until these other storage battery modules reach the charge limits.
On the other hand, when discharging, a storage battery module having a low output voltage and being less charged reaches the discharge limit earlier. If further continuing to discharge this storage battery module, it results in deterioration of the battery characteristics. Therefore, it is necessary to stop discharging when the storage battery module having the lowest output voltage and being the least charged in the battery pack reaches the discharge limit. In this case, it is not possible to continue to discharge the other storage battery modules until these other storage battery modules reach the discharge limits.
That is, when the storage battery modules forming the battery pack have different output voltages and different charge states, it is not possible to fully charge all the storage battery modules during the charging process, and fully discharge all the storage battery modules during the discharging process. In other words, since the storage battery modules forming the battery pack have different battery characteristics from each other, it is not possible to fully use the battery characteristics inherent in the battery pack. Therefore, in order to improve the battery characteristics of the storage battery modules, it is important to eliminate differences in charge states of the storage battery modules forming the battery pack, and to equalize the battery characteristics such as output voltage.
Conventionally, various methods are proposed for eliminating differences in battery characteristics of storage battery modules forming a battery pack (for example, Non-Patent Document 1). Patent Document 1 discloses an equalization device provided with intra-module electric charge equalizing units and an inter-module electric charge equalizing unit. According to Patent Document 1, in a battery pack provided with N storage battery modules connected in series each of which includes M storage battery cells connected in series, each of the intra-module electric charge equalizing units equalizes output voltages of the M storage battery cells, and the inter-module electric charge equalizing unit equalizes output voltages of the N storage battery modules.
The storage battery equalization device disclosed in Patent Document 1 is configured to discharge all the storage battery modules and to charge only selected storage battery modules. That is, all the storage battery modules forming the battery pack equally discharge energy to lower the charge state, and the discharged energy is accumulated in a reactor connected to the selected storage battery module and is used for charging.
By selecting storage battery modules having output voltages lower than an average voltage among the N storage battery modules, it is possible to charge the selected storage battery modules and discharge the other storage battery modules, thus equalizing voltages of the storage battery modules forming the battery pack.